See http://jot.andstuff.org/.  Some highlights:

*Full XHTML compliance.
*Written in Python.
*Uses a home-grown memory-mapped object store as the PageDatabase.  This is not only fast, but it allows the code to be simplified greatly, as objects in the store require no instantiation.
*A WikiPedia-style diff engine (also donated to MoinMoin).
*Rudimentary support for authentication and access control lists to control permissions for pages and page trees.  Fully-open wikis with no UserName authentication are supported as a trivial subcase of this functionality.

This engine is presently in use at AndStuffWiki.

----

The Jotwiki -- see http://www.jotwiki.net/ .
Some highlights:

* Written in Java.
* source code is GPL and can be downloaded from sourceforge.
* stores wiki page data in text files.
* support for access control lists and CAPTCHA.

----
CategoryWikiImplementation